Aftershocks hazard in Italy Part I: Estimation of time-magnitude distribution model parameters and computation of probabilities of occurrence

We analyzed the available instrumental data on Italian earthquakes from 1960 to 1996 to compute the parameters
of the time-magnitude distribution model proposed by Reasenberg and Jones (1989) and currently used to make
aftershock forecasting in California. From 1981 to 1996 we used the recently released Catalogo Strumentale dei
Terremoti ‘Italiani’ (CSTI) (Instrumental Catalog Working Group, 2001) joining the data of the Istituto Nazionale
di Geofisica e Vulcanologia (INGV) and of the Italian major local seismic network, with magnitude revalued
according to Gasperini (2001). From 1960 to 1980 we used instead the Progetto Finalizzato Geodinamica (PFG)
catalog (Postpischl, 1985) with magnitude corrected to be homogeneous with the following period. About 40
sequences are detected using two different algorithms and the results of themodeling for the corresponding ones are
compared. The average values of distribution parameters (p = 0.93±0.21, Log10(c) = –1.53±0.54, b = 0.96±0.18
and a = –1.66±0.72) are in fair agreement with similar computations performed in other regions of the World. We
also analyzed the spatial variation of model parameters that can be used to predict the sequence behavior in the
first days of future Italian seismic crisis, before a reliable modeling of the ongoing sequence is available. Moreover
some nomograms to expeditiously estimate probabilities and rates of aftershock in Italy are also computed.